The present invention generally relates to angular accelerometers (i.e., rotational acceleration sensors) and, more particularly, to a balanced microfabricated angular accelerometer.
Accelerometers are commonly employed to measure the second derivative of displacement with respect to time. In particular, angular accelerometers measure angular acceleration about a sensing axis. Angular accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to the sensed angular acceleration for use in vehicle control systems. For example, the sensed acceleration signal may be used to determine a potential vehicle rollover event and to control automotive devices in response thereto. Angular accelerometers may also be used to control a disk drive read/write head such that a control system associated therewith may compensate for severe shock and/or vibrations that cause the angular acceleration.
One approach to determining angular acceleration employs an angular velocity sensor to sense angular velocity, and differentiates the sensed angular velocity to determine the angular acceleration. The design for an angular velocity sensor is generally complex, and angular velocity sensors are typically expensive to produce. In addition, acceleration measuring devices employing an angular velocity sensor typically require a differentiator which adds to the complexity and overall cost of the device.
Another approach for determining angular acceleration uses a combination of two linear accelerometers mounted to a rigid body for sensing linear acceleration along two respective perpendicular axes. Generally, the linear accelerometers each employ a mass suspended from a frame by multiple beams. The mass, beams, and frame act as a spring-mass system, such that the displacement of the mass is proportional to the linear acceleration applied to the frame. The signal extracted from two linear accelerometers can be used to extract angular acceleration information. Linear accelerometers are readily available and easy to use; however, in order to measure angular acceleration while rejecting linear acceleration, the scale factor, i.e., sensitivity or gain, of the two sensors generally must be matched.
A further approach for an angular accelerometer is disclosed in U.S. Pat. No. 5,251,484, entitled xe2x80x9cROTATIONAL ACCELEROMETER,xe2x80x9d which employs a circular hub centrally supported on a substrate and connected to radially disposed thin film spoke electrodes that flex in response to angular acceleration. Rotational acceleration measurement is achieved by using a differential, parallel plate capacitive pick-off scheme in which the flexible spoke electrodes at the periphery of the fixed disk rotate between fixed reference electrodes so that an off-center position of moving electrodes results in a measured differential voltage from which the disk motion is determined. The sensing capability for such an accelerometer is generally limited to the amount of movement of the flexible spoke electrodes. This cantilevered design with rotary electrodes generally requires high structural matching to ensure predictable gain, phase, and linearity response. The linear and cross-axis sensitivity (gain) is highly dependent on the structural matching. Additionally, separate input and output contacts for each capacitive plate add to the overall complexity and cost of the accelerometer.
More recent designs of angular accelerometers are disclosed in U.S. application Ser. No. 09/410,712, filed on Oct. 1, 1999, and U.S. application Ser. No. 09/782,708, filed on Feb. 13, 2001, both assigned to the assignee of the present application. The microfabricated angular accelerometers disclosed in the aforementioned U.S. patent applications have a rotational inertial mass formed on a substrate and suspended over a cavity via a plurality of support arm tethers. Such accelerometers achieve enhanced sensitivity over previously known accelerometers. However, the design of some angular accelerometers may result in poor linear cross-axis sensitivity on at least one axis, particularly for accelerometers having an asymmetric structure.
Accordingly, many conventional angular accelerometers often suffer from various drawbacks including errors introduced by cross-axis accelerations. It is therefore desirable to provide for a low-cost, easy to make and use, enhanced sensitivity angular accelerometer that eliminates or reduces the drawbacks of the prior known angular acceleration sensing devices, including enhancing the sensitivity of the sensor to structural asymmetries, fabrication processing, packaging, impulsive shocks due to handling, and temperature-induced stresses.
In accordance with the teachings of the present invention, an angular accelerometer having a balanced inertia mass is provided. The angular accelerometer includes a substrate, a fixed electrode supported on the substrate and including a first plurality of fixed capacitive plates, and a rotational inertia mass suspended over a cavity and including a plurality of movable capacitive plates arranged to provide a capacitive coupling with the first plurality of fixed capacitive plates. The angular accelerometer also includes a central member fixed to the substrate and located substantially in the center of the rotational inertia mass. A channel is formed in the rotational inertia mass, and a signal line extends within the channel formed in the rotational inertia mass. A plurality of support arms extend between the central member and the rotational inertia mass for supporting the rotational inertia mass relative to the fixed electrode and allowing rotational movement of the rotational inertia mass upon experiencing an angular acceleration. The angular accelerometer has one or more openings formed in the rotational inertia mass so as to balance the rotational mass to compensate for the channel and provide a center of mass substantially centered about the rotational inertia mass.
In the disclosed embodiment, an input is electrically coupled to one of the fixed electrodes and the rotational inertia mass for receiving an input signal, and an output is electrically coupled to the other of the fixed electrode and the rotational inertia mass for providing an output signal which varies as a function of change of the capacitive coupling and is indicative of angular acceleration. The balanced rotational inertia mass equalizes the frequencies of the orthogonal mode, which in turn significantly improves the cross-axis response to the structure. The balanced inertia mass further offers a processing benefit in that the one or more etched openings formed in the inertia mass may facilitate cavity venting prior to the release of the structure.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.